1. Field of the Invention
The invention is generally related to reinforced materials and more particularly to materials that are reinforced through the use of nanostructures to be super strong materials.
2. General Background
In the present state of the art, reinforced composites use particles or fibers which have micron-sized diameters. Fiberglass and carbon fiber composites are examples of fiber-reinforced composites. Typical fibers used in state-of-the-art composites have diameters greater than 0.0001-inches (2.54 microns) to 0.005-inches (30.4 microns), and length/diameter (L/D) ratios greater than 1 micron. Carbon fiber reinforcements are typically 5 to 8 microns diameter and grouped into tows or yarns of 2,000 to 12,000 fibers. The fiber modulus can range from 207 GPa to 960 GPa.
A typical fiber-reinforced material combines the properties of the fiber material with those of the matrix material in which the fibers are embedded. A variety of combinations of fiber and matrix materials are used; e.g., glass, carbon, and ceramic fibers are used with epoxy resin, glass, metal, ceramic, and carbon matrix materials. A composite material is one in which two or more materials that are different are combined to form a single structure with an identifiable interface. Typically, a composite material is formed from a matrix material (such as metals, ceramics, or polymers) with reinforcing materials as particles or fibers (such as ceramics or carbon and fillers). The new structure of the composite material has properties that are dependent upon the properties of the constituent materials as well as the properties of the interface. A composite material offers properties that are more desirable than the properties of the individual materials. Whereas the two or more contributing materials in a composite material retain their own distinctive properties, the new composite material has properties which cannot be achieved by the individual components alone. More narrowly, a composite material is composed of single or hybrid reinforcement materials embedded in a matrix material.
Composite materials typically form molecular bonds in which the original materials retain their identity and mechanical properties. They can have very selective directional properties. In comparison, metal alloys form bonds at the atomic level to produce homogenous materials that have isotropic properties (the same in all directions).
An example of a polymer composite or fiber reinforced polymer (FRP) composite is a thermoset or thermoplastic polymer matrix reinforced with fibers. The FRP composites are composed of resins such as polyesters, vinyl esters, and phenolics with reinforcements such as glass fibers.
A metal matrix composite (MMC) combines into a single material a metallic base with a reinforcing constituent, which is usually a non-metallic such as a ceramic or carbon fiber. Combining two pre-existing constituents with commonly used processes such as powder metallurgy, diffusion bonding, liquid phase sintering, squeeze-infiltration, or stir-casting generally produces composites. Highly reactive metals are typically formed by in situ chemical reactions within a precursor of the composite. MMCs have several distinct classes, generally defined with reference to the shape and size of the reinforcement constituent, such as Particle-Reinforced Metal Matrix Composites, Short-Fiber and Whisker-Reinforce'd Metal Matrix Composites, Continuous Fiber-Reinforced Metal Matrix Composites, Monofilament-Reinforced Metal Matrix Composites, Interpenetrating Phase Composites, and Liquid Phase Sintered Metallic Composites. Particle-Reinforced Metal (PRMs) Matrix Composites contain approximately equiaxed reinforcements, with an aspect ratio of less than about 5. The reinforcements are typically ceramic such as SiC or Al2O3. These are produced by solid state (e.g., powder metallurgy) or liquid metal techniques (e.g., stir-casting, infiltration). Short-Fiber and Whisker-Reinforced Metal (SFMs and WRMs) Matrix Composites contain reinforcements with an aspect ratio of greater than 5. The SFMs and WRMs are commonly produced by squeeze infiltration. Continuous Fiber-Reinforced Metal (CFRM) Matrix Composites contain continuous fibers, such as Al2O3, SiC, and carbon, with a diameter of below about 20 microns, and are produced by squeeze infiltration with the fibers parallel or pre-woven. Monofilament-Reinforced Metal (MRM) Matrix Composites contain fibers that are relatively large in diameter (typically about 100 microns) and are produced by solid state processes requiring diffusion bonding. Examples include SiC monofilament-reinforced titanium. Interpenetrating Phase Composites have the metal reinforced with a three-dimensionally percolating phase, such as ceramic foam. Liquid Phase Sintered Metallic composites include cemented carbides, in which the carbide particles are bonded together by a metal such as cobalt.
Composites combine the strength of the reinforcement with the toughness of the matrix to achieve desirable properties. The advantages of composites are high strength, high stiffness, low weight ratio, and designed specifics. The composite materials can be separated into three categories based on the strengthening mechanism: i.e., dispersion strengthened, particle reinforced, and fiber reinforced. Dispersion strengthened composites have a fine distribution of particles embedded in the matrix and impede the mechanisms that allow a material to deform (including dislocation movement and slip). Many metal-matrix composites fall into the dispersion strengthened composite category. Particle reinforced composites have a large volume fraction of particles dispersed in the matrix and the load is shared by the particles and the matrix. Most commercial ceramics and many filled polymers are particle reinforced composites. Fiber reinforced composites use the fiber as the primary load-bearing component.
Carbon nanotubes (CNTs) were discovered in 1991 by S. Iijima [S. Iijima, Nature, 354, 56 (1991)]. These large macromolecules are long, thin cylinders of carbon that have unique size, shape, and remarkable physical properties. They can be thought of as a sheet of graphite (a hexagonal lattice of carbon) rolled into a cylinder. They are light, flexible, thermally stabile, and are chemically inert. They have the ability to be either metallic or semi-conducting depending on the “twist” of the tube. Having phenomenal electronic and structural properties, CNTs have been endorsed as the strongest material known to man with an axial Young's modulus >1TPa, a predicted bundled strength of 130 GPa, and the highest strength-to-weight ratio known being 100 times stronger than steel but only one-sixth the weight.
The earliest nanotubes were made from pure carbon. Formed naturally in the sooty residue of vaporized carbon rods, they were an elongated form of fullerene or “buckyball” molecules, clusters of 60 and 70 carbon atoms joined in a graphite-like mesh of hexagonal rings. The first generations were “multi-walled nanotubes” (MWNTs) that consisted of about 5 to 40 single-walled nanotubes (SWNTs) wherein each tube nested inside the other like Russian dolls. A SWNT means the wall of the tube consists of only a single layer of carbon atoms. Later, when scientists began to directly make SWNTs, it was discovered that they could be drawn out to exceedingly long lengths of nanowire without losing any strength or durability.
Bundled SWNT are predicted to have the largest strength-to-weight ratio of any known material, and promise new generations of lightweight, supertough structural materials which could replace metals in the bodies and engines of automobiles, aircraft, and ships, as well as form a new class of energy-efficient building materials. Single-walled carbon nanotubes are also highly thermally conductive, can withstand high temperatures, and are resistant to even strong acids. Finally, SWNT recently exhibited 8 wt. % hydrogen sorption (the highest for any carbon material) which make them desirable for hydrogen storage fuel cells for clean cars of the future.
Carbon nanotubes are almost always coated or partially coated with a thin layer of material, typically carbon, that is just discernible in Transmission Electron Microscope (TEM) images. The surface of a clean nanotube is “slippery”, that is, unlikely to provide an anchor for mechanical reinforcement or is unlikely to be wetted by the matrix material. Because carbon nanotubes are still a new and emerging technology, there is little or no work in the field of using them to produce reinforced materials.